Injector Resistors

However, instead of the flyback board, you may choose to use resistors in series with your injectors.

Several people reported that resistors do NOT result in significantly longer opening times, or any other troublesome effects, so this is a good solution for many installs. To eliminate PWM altogether, use a 5 to 8 ohm resistor (with a 20 to 25 watt rating) in series with each injector.

If you want to avoid using PWM with your low-impedance injectors, you can use ballast resistors in series with the injectors. You should use one resistor (20-25 Watts) in series with each injector, otherwise the injectors may not all draw the same current, and the failure modes become complicated and difficult to diagnose. As well, you would need a very large resistor to handle more injectors. For example, if you allowed 2 Amps through four 1.2 Ohm injectors wired in parallel (0.3 Ohms total) to one 7 Ohm resistor, the power dissipated would be:

If you use resistors that limit injector current to less than 2 amps, you can disable the PWM mode (by setting PWM% to 100%, and time threshold to 25.4msec) and treavolts Requiredt the system as high-impedance. To limit the current to under 2 amps, you need:

For example:

resistor ohms = (14.0 volts / 2.0 amps) - 1.2 ohms

=> resistor ohms = 7.0 - 1.2 = 5.8 ohms

You can also use the calculator below. Enter your injector resistance in ohms, your hold current in amps, as well as your injector impedance in ohms, in the form below and press the "Compute resistor" button.

The 25-watt resistors aluminum case Ohmite resistors (with 1% tolerance) from www.digi-key work well. Below is a picture of a 7.5 ohm resistor, Digi-Key part number 825F7R5-ND.

Ohmite has several suitable resistors, with part numbers that start 825F (25 Watt, aluminum case with mounting ears) and end in XRY, where X and Y indicate X.Y ohms. Depending on injector, pick 2-8 ohms or so.

You may be able to use less resistance to protect the flyback components - just a few ohms combined with PWM may do the trick. Be sure to use one resistor in series with each injector, and then you can parallel these into the two banks. Do not share two or more injectors per resistor, use a resistor per injector.

There a lower bound to the pulse width, below which a low impedance injector cannot be expected to reliably function. There are two problems with running the very lower pulse widths that result from large injectors. There is a limit on the physical ability of the injector to opening and close as quickly as possible, and there is also a limit to the ability of the Mega Squirt controller to adjust the pulse width to an optimum value at very low pulse widths.

The absolute physical limit depends on your particular injectors and the hardware that controls them. Some are able to go as low as 1.1 to 1.5 milliseconds [ms]. Note that there are three components to the injection duration - the opening time, the commanded pulse, and the closing time. Ideally you would want the opening and closing times to be a short as possible to have the controller determining as much of the amount of the time injected as possible. The opening time is difficult to adjust given a certain operating voltage. The closing time, however is controlled to a degree by the flyback circuit in the Mega Squirt.

With very large injectors [for a given application], the idle pulse widths may be around 1.0 millisecond. This is a problem because in the standard code for Mega Squirt, the resolution of the steps is 0.1 ms. So a 1.1 millisecond “squirt” will only be able to be adjusted in ~9% increments (i.e. 1.0, 1.1, 1.2 etc.), which may be too coarse to get a good idle. The high-resolution Mega Squirt code can help in this situation, but you lose the PWM current limiting mode so you have to run resistor packs with peak and hold low-impedance injectors.

An ideal idle duration is around 2.3 ms, and this is approximately where properly sized injectors should operate. This gives good resolution [~4%], and you should be able to get a real good idle.

You will need to acquire connectors for wiring the Mega Squirt to your injectors, etc. Niehoff has individual injector connectors under part number 28419 (connector) and 28418 (sealing boot). On the web, Waytek has lots of different connectors that you can use in building your Mega Squirt. Their prices are about as cheap as you can find. The injector connectors are AMP part number 827551-3, but sometimes you have to buy a large quantity. Also try DelCity. They are not quite as cheap, but they may have stuff you cannot get from Waytek.

Injector Bungs

You can get information on injector bungs for port injectors by checking out www.sdsefi.com for injector/manifold installation information, along with lots of other great information. The bungs are 0.530"-0.535" inside diameter [about 17/32" or 13.5 mm]. The fuel supply lines for the top of the injectors are the same size.

VERY IMPORTANT! If you do not own at least two fire extinguishers, go buy some right now! Experimentation with EFI can be very dangerous because you are playing with high pressure gasoline. Install at least one fire extinguisher in your work area (away from where the fire is mostly likely to occur) and carry another one in your car. Do not ignore this advice. We do not want to be visiting you in the hospital or worse!

MSD and others have an “Epoxy-In Pocket” fuel injector bung as PN 2120 (set of 8). Holley also offers them as PN 534-83 for a four pack (~$50), 534-84 for a pack of six (~$72), or 534-85 for a pack of eight (~$94).

These bungs can be held in place with epoxy or welded and is used for fixed fuel rail systems only. These bungs are CNC-machined from aluminum for precise dimensions and have a ¾” OD. Internally, the pockets are contoured to accept the bottom sealing O-ring of a standard injector. MSD also has “Thread-In Pockets”. The aluminum pockets will screw into a ¾”–16 hole and are supplied with a #8 O-ring to seal the pocket to the manifold. PN 2125 gets a set of 8.

Fuel Rails

Most injector systems will use one or more fuel rails. These serve two functions: they supply fuel to a multiple number of injectors (4 on a 4 cylinder, for example), and they physically locate the tops of the injectors. Most OEM rails can be made to work with standard engine configurations, but if you are doing a custom conversion you may have to fabricate fuel rails. Many place supply blank aluminum fuel rail extrusions in whatever length you need. One example is Ross Machine. They have two styles of fuel rail extrusion. They can also create custom fuel rails for you, with the injector holes placed to suit you.

The aluminum extrusion comes in two sizes:

• Dash 10 (.800" Bore) - $10/foot, and
• Dash 6 (.500" Bore) - $12/foot.

For fabricating fuel rails, MSD has “Fuel Delivery Top Mounts”, PN 2115, set of 8. These fuel delivery mounts are CNC machined from #304 stainless steel for great durability and precise dimensions. They slide over ½” steel tubing (MSD PN 2205) then are brazed or TIG welded in place to form a fuel rail. Fuel is routed through a 5/16” hole aligned to the mount and the injector. The PN 2105 Fuel Rail Clip is required for assembly. Their “Stainless Steel Fuel Tubing”, PN 2205, comes in 2 four feet lengths of 304 stainless steel tubing, and is perfect for making custom fixed rails. The seamless tubing has a ½” OD and .035” wall.

Throttle Bodies

Your throttle body choice depends on whether you are going to use throttle body injection or port injection.

Your throttle body needs to do 2 things:

1. control the amount of air going into the engine, and
2. report the throttle position to Mega Squirt via a TPS.

For port injection, you can convert an existing carb, to do both jobs - the carb already controls the air flow, you have to adapt a TPS sensor to it. You may choose to machine out the venturis, remove the float bowls and fuel circuits if you wish, but that isn't necessary (but may be desirable for a number of reasons, including increased power!).

Some people use the complete individual runner (IR) throttle body and injector set-ups off late model motorcycles - they often have enough flow for automotive engines, and are frequently available cheaply on eBay.

However, if you are planning on a throttle body injection set-up, you need a dedicated TBI unit (in order to supply the fuel to the injectors, etc.), which can be hard to find for larger engines - Holley has made a 4bbl TBI for years (in 650, 700 and 900 cfm sizes), and as the computer fails regularly on these, they are sometimes available separately on eBay. TBIs have the advantage of having the fuel pressure regulator built in, of course.

Note that for either port or throttle body injection you can use multiple throttle bodies to support your power levels, if your manifold configuration can be adapted for them.

However, if you are uncertain of your throttle body's application, you can measure the throttle bore size. However, you can't really compare the throttle of an EFI throttle body to the throttles of a carb. This is because the throttle(s) of a EFI TB is the main restriction, but on a carb, it is the venturis that are the main restriction. So you really have to compare the EFI throttle size to the carb's venturi size. However there are also a number of other considerations, such as that you can go larger with a EFI TB than a carb without suffering so many adverse effects because a vacuum signal isn't needed for the EFI to operate.Fuel delivery is always good with EFI (well, mostly). When selecting a throttle body, there are a number of considerations. You need it to flow enough to support your engine's horsepower (or more correctly, to not restrict your engines power). Generally, you want to take the throttle body from an engine that made similar horsepower to your engine.

However, there are some drawbacks to a too large throttle body:

• At low rpm, you go from low kPa to 100kPa with very little throttle movement, making driveability 'worse'. For example, with a very large throttle body you may get 100 kPa at 20% throttle at 2000 rpm. This means if you want to hold it at 40 kPa for cruise, I have to be very steady on the throttle, as small movements may produce large changes in engine output (so it's harder to be smooth), and
• A small throttle movement (and a small V/sec TPS signal change) can result in a very large change in MAP (as mentioned above) at low rpms. The result is little (or no) accel enrich when the engine needs it most. However, you can usually tune around circumstances like this by richening the VE table at low rpms and higher kPa (say < 2500 rpm and> 70 kPa) by about 5-7%. This has a negligible affect on fuel economy, since you likely never see 70 kPa while cruising.

For reference, the GM tuned Port Injection engines used throttle bodies with two 48mm throttles. These support about 230 horsepower, however these throttles were not the limiting factor in the power produced by these engines.

To calculate how much horsepower you can make from a given throttle body size, you can use the estimator below:

Fuel Supply System

In order to use Mega Squirt, you will have to implement a high-pressure fuel supply system. You MUST understand how to do this properly, and this manual DOES NOT include everything you need to know. If you are unsure about your installation, have a qualified mechanic look it over before attempting to start your vehicle.

Fuel Pumps

You will need a high pressure pump with enough volume at your operating pressure to feed you engine under maximum load. Typical pressures needed in the neighborhood of ~45 psi for port fuel injection, ~10-20 psi for TBI injection. A port injection pump will work with TBI, but not vice-versa.

OEMs usually place pump inside the fuel tank. In an EFI retrofit it is generally easier to use an external fuel pump. Ford used external fuel pumps on 1989 era 150 trucks which may be a candidate for use. These are high pressure [port EFI] pumps that will work in most applications. Econoline vans have these as well.

The external pumps used in Ford F150 fuel injected trucks from the 89-93 model years are Delco EP286. At 12 volts, the operating pressure is 70-95 PSI with 36-40 gals per hour. The biggest Delco pump is the EP424, which is 75-90 PSI at 40 gals per hour. EP 268 is a GM# 25117086, EP 424 is a GM# 25176156."

Here is a picture of the Econoline pump:

The Carter pump #P70199 (the outlet is 7/16 standard pipe thread and the inlet is 15/32 clamped hose type fitting or 3/4 standard thread. The specs are 95-PSI max, 68-93 G/Hr wide open). This is the highest flowing Carter external fuel pump in the book. It will produce up to 95 psi, and crosses over to EP7107 at Kragen for about $80 (unfortunately one end does not come off like the Carter). You might want the Ford style pump EP7109($80). You will need this if you want to be able to modify ends to be 3/8".

Others have had luck using the external pump from various fuel injected VolksWagen models (87 VW Fox, for example). Part number is: Bosch 0 580 254 957 reportedly rated at 90 GPH@ 70PSI, you might find them for about $130 new from www.germanautoparts.com. This pump consists of a fuel pump, filter, and an "accumulator". You can leave the accumulator in place since it does not affect the running volume or pressure, and on used pumps they are often rusted so you might not want to mess with it.

Auto Performance Engineering has many high volume Walbro pumps (and their specifications) on their site.

Fuel Line

Steel tubing is recommended, but you MUST have short sections of rubber line in the feed and return lines between the engine and frame to allow for engine movement. The return line should have minimal restriction. For reference, GM systems typically have 3/8" feed lines and 5/16" return lines.

You may be able to use your original fuel line as a return line, plumbing a new 3/8" (10mm) line for fuel supply. You can run the return line into the tank, or reroute it to a fitting or nipple you install in the fuel tank filler neck/tube assembly (in which case you may be able to use the original pick-up for your supply line). If you run a new pick-up into the tank, it will need a filter. GM sells a sock-type filter that is a good fit for 3/8" lines. It is part number 5651702 and costs about $15.

You may have to fabricate fuel lines for your system. Tubing is available in steel, stainless steel, and aluminum for this purpose. The size is generally given as the outside diameter of the tubing. Unless you have a very unusual combination (or very high horsepower, well over 500+), you should be able to use 3/8" tubing for both the supply and return lines.

Buy a good tubing bender (there are numerous styles in various price ranges) so that you don't kink or collapse the tubing while bending it. (You can also bend it over a V-belt pulley, in some cases.)

The AN (Army-Navy) 'dash' system of hose and fitting sizing was established many years ago by the American military as a common measurement for hoses and fittings. It designates the outside diameter of the metal tube that is compatible with each size of fitting. The AN dash measure is the standard for performance hose applications. These dash sizes are expressed in 16^th of an inch. For example, an -06 fitting is ⁶/₁₆ of an inch or ³/₈", just right for our fuel lines!

Most fittings and adapters in the automotive aftermarket are based on a 37° sealing angle (SAE J514 37° - formerly known as JIC). These are also often referred to simply as AN fittings. Male and female 37° fittings will mate together for a leak-proof connection.

Be aware that there are other similar fittings and adapters that use a 45° sealing surface (SAE J512), such as those commonly available at your local hardware store for flared copper pipe. These 45° fittings and adapters can also be found in some OEM automotive applications. However, although they may look very similar to a 37° fitting, they are not interchangeable. In some sizes, they may thread together (-02, -03, -04, -05, -08, -10), but will not seal properly, due to the difference in sealing surface angles. Be sure you know the sealing angle of the fittings you are connecting!

Abrasion (the rubbing of the hose against some other component) is the number one cause of hose failure. A leaking fuel hose can start a very dangerous fire in your car, so make sure hose assemblies are routed properly to reduce the chance of any abrasion damage. Use a support every 12 to 18 inches (30 to 45 cm) to secure the hose. For chafe protection, be sure to install a grommet at any point a hose passes through a panel or bulkhead.

Besides steel or aluminum tubing fuel line, you can also use one of the steel or nylon braided hoses from various suppliers. Generally these use the same AN 'dash' sizing system, and can use appropriate fittings to connect to 37° flare, NPT thread, or other systems.

Note that if you are using a factory fuel rail, you may be able to find an aftermarket adapter to mate your OEM fuel fitting to an AN hose. For example, Accel offers TPI fuel rail fittings (pn 74730, ~$32) for -06 hose that will fit most General Motors TPI fuel injections systems.

If you need a simple way to get to a barbed fitting to connect up rubber EFI hose to the General Motors 2 bbl TBI, your local auto parts house probably stocks GM fuel line repair kits in the HELP section. These consist of 9" of steel fuel line in 3/8" and 5/16" outside diameter with an O-ring and Saginaw fittings 14/16 mm, respectively, on one end and a barbed end crimped on the other. The steel lines are about $4.00 each. These pieces thread into the steel adapters on the GM Rochester TBIs. For a complete listing of various fittings with part numbers, etc. try:

http://www.ag.auburn.edu/users/gparmer/efi/fittings.txt

IMPORTANT: Keep the fuel lines out of passenger compartment and routed safely away from moving or hot parts to avoid damage/excessive heat. For flexible rubber hose use the SAE 30R9 EFI hose which is rated at 250 psi. EFI hose clamps are also recommended rather than gear clamps. Check with someone who knows if you are not sure about your installation. Nobody needs a 50 psi gasoline fed fire to ruin their day!

Note that if you feel your fuel supply is not smooth enough, you can add an accumulator. OEMs often have nice small diaphragm types, or you can plumb in a vertical section of rubber hose tee'd off the supply line (and plugged at the top end). This traps air and uses it to cushion the fuel pressure.

Here is a GM style accumulator (it is about 2" (50mm) long and uses 3/8" tubing):

Or you can make your own:

Fuel filter

Use a fuel injection fuel filter rated for the pressure at which your system operates. DO NOT use a universal carburetor filter - the higher pressure of fuel injection systems may cause it to burst! Position the filter downstream of the pump so that a clogged fuel filter will not over heat the fuel-cooled pump.

Fuel Pressure Regulator

The vacuum referenced fuel pressure regulator is essential. It provides constant pressure differential between fuel at injector nozzle and manifold air pressure [port EFI] or atmospheric pressure [TBI]. This makes the injected fuel quantity solely a function of the injector open time.

If you were to 'cap off' the manifold vacuum port on the fuel pressure regulator, you are reducing the dynamic range of the injectors. This means you will need lower pulse widths at at (giving less control over idle mixtures) and lower flow under boost (restricting the maximum horsepower).

So, in general, for port injectors, have the fuel pressure regulator connected to the manifold vacuum is a good thing. There is very little reason not to do it (though some have argued against it for individual runner port EFI set-ups).

If you have an adjustable fuel pressure regulator (FPR), set the pressure with the fuel pump running, but the engine not running - that's your base fuel pressure (it is referenced to atmospheric pressure).

The regulator is typically at the far end of the fuel rail (after the injectors), but performs its job anywhere, so long as it is after the fuel pump. However, if you have the regulator before the rails, then the full volume of fuel isn't circulating through the rails. Only the amount of fuel actually injected moves, and the fuel can get quite hot, which may require special injectors, etc. Apparently OEM use special injectors, etc., with returnless systems, which is essentially what one has if they put the regulator before the injectors. This can also create problems trapping air on assembly that can cause issues at first start-up.

If you are using an aftermarket fuel pressure regulator, it is a good idea to also install a pressure gauge, since most of these are adjustable. For TBI, use a 0-30 psi gauge. For port injection use a 0-60 psi or 0-100 psi gauge. Most of these gauges will mount directly on a fuel fitting using a ¹/₈" NPT thread. These are available from most aftermarket speed parts suppliers, such as Summit Racing or Jegs.

Surge Tank

You only need a surge tank if you are using a low pressure pump to supply an external high-pressure pump. Some pumps come with an accumulator after the pump, and these can be left in place.

Wiring the Fuel Pump

To activate the fuel pump, Mega Squirt provides a ground for the fuel pump relay circuit on pin 37. The relay is wired for 12 volts switched from the ignition switch, and the relay is grounded through Mega Squirt [pin 37 on the DB-37 connector].

Mega Squirt will disable the fuel pump when RPM = 0 and enable while non-zero (cranking/running), except for Version 2.00 (and up) embedded software which will perform a short priming pulse, then shut down the pump if the engine is not running after 2 seconds.

You might want to consider a safety switch in the fuel pump circuit when installing an electric fuel pump. Holley has one (12-810, ~$20) that will ensure the fuel pump will not run unless the engine has oil pressure. It stops the pump from running if the motor stalls with the ignition on. Wiring the switch through the starter solenoid circuit energizes the pump on engine start-up. Once the engine has started, the switch continues to provide power to the pump as long as there is oil pressure to keep the switch turned on.

Note: An inertial safety shut off switch should be installed and used to kill power to the pump upon significant impact to vehicle.

These switches are available in junk yards from EFI Fords. The switch is on the drivers side in the trunk, near the trunk hinge, mounted so that it is between the interior bracing and the rear quarter panel (protected from being knocked around if you stuff your trunk full of stuff). It is mounted with the reset switch straight up. Note switch mounting orientation probably matters.

It is Ford Part # F2AB-9341-AA. The wire going into it is about 14 gauge, so it should be capable of handling the full current of the fuel pump.

The markings on the switch show that it has NO/NC (normally open/normally closed) positions so that it should be able to accommodate any possible fuel pump configuration.